Modal noise in multimode fibers under restricted launch conditions

Modal noise is a non-Gaussian noise source associated with the interference of fiber modes at a detector. Most derivations of the statistics of modal noise assume a limiting case of overfilled launch conditions where there are many modes propagating in the fiber. However, if the transmitted power is contained in a few modes, commonly referred to as restricted-mode launch conditions, the general assumptions for modal noise theory are not valid. Here, we present experimental measurements of the noise distributions for both full- and restricted-mode launch cases and show that the noise statistics for a restricted launch are significantly different than for a full-mode launch. In particular, the variance of the noise distribution is larger than in the over-filled launch case leading to a degradation in the signal-to-noise ratio (SNR). Modifications of existing theory are then used to develop a numerical model for the noise statistics of a restricted-mode launch and good agreement between this model and experimental data is achieved.